The design and operating performance of multi-media filters are determined by a series of parameters, which can be mainly divided into design parameters and operating parameters:
Design parameters (determine the basic specifications and capacity of the equipment)
Filter media (media) types and grading:
Common media: Anthracite, Quartz Sand, Manganese Sand (for iron and manganese removal), Magnetite, Activated Carbon (also takes into account the adsorption of organic matter, but strictly speaking, activated carbon filters are another type), Garnet, etc.
Grading principle: The upper layer uses a medium with a larger particle size and a smaller density (such as anthracite), and the lower layer uses a medium with a smaller particle size and a higher density (such as quartz sand and magnetite). Typical configuration:
Upper layer: anthracite (particle size 0.8-1.6mm or 1.2-2.0mm, thickness 300-500mm)
Middle layer: quartz sand (particle size 0.4-0.8mm, thickness 300-400mm)
Lower layer: quartz sand or magnetite/garnet (particle size 0.2-0.4mm or smaller, thickness 100-200mm) – This layer mainly plays a supporting and fine filtering role.
Function: The large particle size and low density layer intercepts large particles, and the small particle size and high density layer intercepts small particles, realizing step-by-step filtration, making full use of the filter bed depth, and improving the interception capacity and filtration cycle.
Total filter layer thickness: Usually between 700mm – 1200mm. Thicker filter layers mean greater dirt holding capacity and longer operating cycles, but the equipment height and cost also increase.
Filter diameter: Determines the equipment’s processing flow capacity. Common specifications range from Φ600mm, Φ800mm, Φ1000mm, Φ1200mm, Φ1600mm, Φ1800mm, Φ2000mm, Φ2400mm, Φ3000mm and even larger.
Design flow/processing capacity: Unit m³/h, determined by the total system demand and the number of filters.
Design pressure: Usually 0.6MPa (6 bar) or higher (such as 1.0MPa), which must meet the system working pressure and backwash pressure requirements.
Water distribution system:
Upper water distribution: Usually a bell-mouth or porous tube water distributor, requiring uniform water distribution.
Lower water distribution/water collection system: The most critical is the strainer nozzle or dome plate + water cap. Requirements:
Evenly collect filtered water.
Evenly distribute backwash water and air.
The most critical parameter: gap width (or opening aperture): must be smaller than the minimum medium particle size (usually refers to the finest medium in the bottom layer) to prevent filter media from losing. Common gap widths are 0.25mm – 0.4mm. The materials are mostly SS304 or SS316 stainless steel, ABS, reinforced nylon, etc.
Backwash drain: Located above the filter layer, it is used to evenly discharge wastewater and impurities washed out of the filter media during backwashing. Its height and position must ensure that the filter media will not be lost during backwashing.
Shell material: carbon steel lining rubber (most common and economical), stainless steel (SS304/316L, used for food and medicine or high corrosion resistance requirements), fiberglass (FRP, lightweight and corrosion-resistant).
